The present invention relates generally to gas turbine engines and, more particularly, to an apparatus for passively controlling the axial position of an inner casing within the compressor or turbine section of a gas turbine engine based on flowpath pressures during different modes of engine operation as well as using this method of control to advantageously adjusting a gap clearance between adjacent rotating and non-rotating components.
As one of ordinary skill in the art will appreciate, the efficiency of a gas turbine engine is dependent upon many factors, one of which is the radial clearance between adjacent rotating and non-rotating components, such as, for example, the rotor blade tips and the casing shroud surrounding the outer tips of the rotor blades. If the clearance is too great, an unacceptable degree of working fluid leakage will occur with a resultant loss in efficiency. If the clearance is too little, there is a risk that under certain conditions contact will occur between the components and cause damage thereto.
The potential for contact between rotating and non-rotating components may be present over a range of engine operating conditions. For example, one such condition is when the engine rotational speed is changing, either increasing or decreasing, since temperature differentials across the engine frequently result in the rotating and non-rotating components radially expanding and contracting at different rates. For instance, upon engine accelerations, thermal growth of the rotor typically lags behind that of the casing. During steady-state operation, the growth of the casing ordinarily matches more closely that of the rotor. Upon engine decelerations, the casing contracts more rapidly than the rotor. These type of issues are also present during both startup and shutdown procedures, as it is often difficult to match the casing to rotor thermal growths during these operations.
Control mechanisms, usually mechanically or thermally actuated, have been proposed in the prior art to maintain or reduce blade tip clearance so that leakage is minimized. However, none represent an optimized or efficient design. Specifically, active control systems require feedback loops, control systems, extra components and, thereby, add cost to the machine. It will be appreciated that, if passive systems could provide similar results, they would be desirable due to their more simplified activation strategy, which typically requires fewer parts, less cost, and greater robustness. Consequently, a need still remains for an improved mechanism for clearance control that maintains a narrow tip-shroud clearance through the operational range of the engine so to improve engine performance and reduce fuel consumption. Additionally, it will be appreciated that conventional methods and systems for axially positioning the inner casings typically are present through the compressor and turbine sections of the engine are similarly deficient, and that there would be commercial demand for improved methods and systems for controlling the axial position of these structures. As will be appreciated, such methods of control, if made cost-effective, robust and efficient, may be put to other uses than the specific exemplary ones described herein.